window.onload = function(){
//var url = "a.jpg,b.jpg,c.jpg".split(',');
var url=[];
url[0] = 'top_bg01.jpg';
url[1] = 'top_bg02.jpg';
url[2] = 'top_bg03.jpg';
var n = Math.floor(Math.random() * url.length);
var elm = document.getElementById('main');
elm.style.backgroundImage = 'url(common/jpg/' + url[n] + ')';
}